Plugging an oil well is a common cementing operation practised for a variety of reasons, for example, to sidetrack above a fish (equipment lost in the hole), to initiate directional drilling in a weak formation, to plug back a zone or a complete well for abandonment, to cure a lost circulation problem encountered during drilling, or to provide a test anchor when a weak formation exists in an open hole below the zone to be tested.
In all these cases the cement plug is often required to fill a zone at a depth considerably above the bottom of the well, so a deep "rathole" exists below the cemented interval. In these circumstances problems are frequently encountered and several attempts may be required to achieve a successful plug. One important reason for this is that the cement is often considerably denser than the drilling fluid initially in the wellbore. In this situation the cement plug is unstable and it can fall through the drilling fluid, contaminating the cement and resulting in a failed plug.
Several methods have been proposed in the past for supporting cement plugs, none of them gave entirely satisfactory results. In an article by R. C. Smith et al, Journal of Petroleum Technology, November 1984, 1897-1904, the setting of a viscous pill below the desired location of the bottom of the cement (BOC) is suggested. The pill can be made by adding a viscosifier such as bentonite to the existing mud system or by formulating a viscous spacer using, for example, MUDPUSH XL (Trademark of Schlumberger Dowell). However, experimental work has shown that this approach is not always reliable and that the cement can still channel through the viscous pill.
An alternative to the viscous pill is the reactive viscous pill (RVP) which contains an additional component which reacts with either the drilling fluid initially filling the well or any cement which starts to fall through the pill. Examples of the RVP method are described by D. L. Bour et al. SPE 15008 (1986), 187-193. The reactive component is usually sodium silicate which reacts with calcium ions to form a stiff gel. This method relies on good mixing between the RVP and the cement or drilling fluid and this is difficult to achieve in such a way that the gelled region extends across the entire cross-section of the wellbore. Indeed, the placement procedure conventionally employed minimises mixing during placement and it is claimed that the reaction is initiated by the cement falling through the RVP material.
It is therefore an object of the invention to improve the known methods used for placing a plug in a wellbore so as to prevent failure of the plug. It is a more specific object of the invention to improve the viscous pill method such that the resulting pill has improved gel strength properties.
Furthermore of interest with regard to the present invention, are fluids which form a stiff gel, without the need for mixing with a second fluid downhole. These fluids are used in fields of the oil industry, for example for lost circulation or conformance control. Most of these fluids gel due to a reaction initiated at the time they are mixed on surface and have only a fixed time (usually strongly dependent on temperature) after mixing during which they can be placed in the appropriate place in the wellbore.
One class of these fluids thickens due to a chemical reaction initiated by subjecting them to a high shear stress. This shear may be provided by a nozzle at the end of the drill pipe used to place the fluids in the wellbore. U.S. Pat. No 4,663,366 discloses an emulsion wherein the oil phase contains a water swelling hydrophilic clay such as bentonite which is initially kept separate from the water phase by a membrane or film of polymeric material. This membrane is formed by a polyamine derivative dispersed in the oil phase and a polyacrylamide and a polycarboxylic acid dissolved in the water phase. When the emulsion is subjected to high shear forces the membrane is ruptured and the bentonite is brought into contact with the water at which time it swells and thickens the liquid.
International Patent Application number WO 94/28085 discloses an alternative fluid consisting of an emulsion of a continuous oil phase containing an emulsifier and a crosslinking agent for a polysaccharide and a water discontinuous phase containing a polysaccharide. When this liquid is subjected to high shear the emulsion is broken causing the polysaccharide to crosslink and form a gel which is much stronger than that of U.S. Pat. No. 4,663,366. The proposed use for the fluid of Int. Pat. WO 94/28085 was for preventing "lost circulation" or leakage of fluids from the wellbore into the formation and the material was intended to migrate into the formation and then rapidly set, thus preventing further losses of fluid from the wellbore.